Powder actuated tool



May 26, 1970 YuNG sHlNG HSU ETAL 3,5l4,025

POWDER ACTUATED TOOL 3 Sheets-Sheet 2 Filed Oct. 30, 1967 YUNG SHING HSU RAYMOND V POMEROY /NVEA/TORS EUC/(HORN, BLORE, KLRU/S 7' 8 SPAR/(MAN TTORNE'YS May 26, 1970 Filed 0012. 30, 1967.

FICS. 4

YUNG'SHING HSU ETA'- POWDER AGTUATED TOOL 3 SheetS-Sheet 3 YUNG SHING HSU RAYMOND V. POMERQY /NVE/VTORS er Euc/(Ham, 81.09:, KLA/vou/sr a SPARK/mm ArrofiA/frs United States Patent U.S. Cl. 227- 14 Claims ABSTRACT OF THE DISCLOSURE An explosively actuated tool adapted for driving a fastener device into a concrete wall or the like employs a charge or pellet of caseless explosive. The charge or pellet of explosive is placed between a power plug, communicating with the barrel of the tool, and an igniter plug carrying a relatively high resistance electrical conductor for contacting the exterior of the charge or pellet. When the tool is cocked, by urging the barrel thereof against a workpiece, electrical contacts complete a circuit from the aforementioned conductor to a triggerable current source. Operation of the tool trigger then supplies current to the conductor through the aforementioned contacts for raising the temperature of the conductor and the explosive to a value for igniting the explosive.

BACKGROUND OF THE INVENTION Explosively actuated tools, e.g. for driving fastener devices into concrete or masonry walls, conventionally employ a blank cartridge for providing the explosion gases applied to drive the fastener device. A spring-actuated firng pin for such a cartridge, or a spring-actuated adiabatic and/or impact ignition device, requires relatively complicated operating mechanism. Moreover, such mechanism must be of sturdy Construction, requiring appreciable applied force to cock and operate. Furthermore, a blank cartridge, after firng, must be removed or ejected from the tool. There is therefore a need for a simpler, more economical explosively-actuated tool and method for operating the same.

SUMMARY OF THE INVENTION In accordance with the present invention, an explosively actuated tool employs a charge or pellet of caseless explosive which is conveniently received within the chamber of a power plug communicating with the barrel of the tool. An igniter plug, including heater means in the form of a relatively high resistance conductor or wire, is positioned within the chamber in contact with the exterior of this charge or pellet, and when the tool is cocked, electrical connection is completed from a triggerable source of electric current to such high resistance wire. Triggering of the tool provides a current through the wire for a short period of time, raising the temperature rof the wire to a value for igniting the charge or pellet, whereby the gases generated enter the barrel through the power plug and operate the device. It is noted that the explosive material may be inexpensive and includes no igniter means, primer, cartridge, or the like. The arrangement differs from one ernploying a cartridge or the like including an intemal explodable conductor. In the latter case the expense of such cartridge is additionally increased by the embedding of the explodable conductor.

It is accordingly an object of the present invention to provide an improved means and method for explosively driving a fastener device into a workpiece without using a conventional cartridge or firng mechanism.

It is a further object of the present invention to provide an improved method and apparatus of explosively driving a fastener device into a workpiece employing a charge rice or pellet of caseless explosive as the force-providing agency.

It is another object of the present invention to provide an improved explosively-actuated tool including inexpenslve means of firng a charge or pellet of caseless explosive for driving a fastener device.

It is a further object of the present invention to provide an improved electrically operated, powder-actuated tool utilizing a charge or pellet o-f caseless explosive. The subject matter which we regard as our invention 1s particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanyng drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. 1 is a cross-sectional side view of a first powderactuated tool according to the present invention;

FIG. 2 is a side cross-sectional side view of an igniter plug employed in the FIG. 1 device;

FIG. 3 is a transverse cross section of the FIG. 2 igniter plug taken at 3-3 in FIG. 2;

FIG. 4 is a side view partially in cross section of a second powder-actuated tool according to the present invention;

FIG. 5 is a schematic diagram of a first electrical circuit utilized for energizing a powder-actuated tool according to the present invention;

FIG. 6 is a schematic diagram of a second electrical clrcuit utilized for energizing a powder-actuated tool according to the present invention; and

FIG. 7 is a schematic diagram of a third electrical circuit utilized for energizing a powder actuated tool according to the present invention.

DETAILED DESCRIPTION In FIG. l, an explosively operated tool includes a housing generally indicated at 10 including a forward portion 12 which may be designated as a `Ibarrel portion, and a rearward portion 13 which may be designated as a breech portion. Forward portion 12 is provided with a longitudinal bore for slidably receiving a barrel 14 which extends from a muzzle end 16 rearwardly through the housing to a power plug 18 threadably secured in the rear end of the barrel and therefore slidably movable with the barrel. The barrel is provided with an inner bore 20 within which a captive piston 22 is slidable, such piston having an enlarged piston head 24, which slides Within inner Ibore 20, and a ram or plunger 26 adapted to contact and drive the head 28 of nail-like fastener device 30 receivable within the muzzle end of the barrel. Enlarged head 24 of captive piston 22 is provided with a piston ring or seal 32 therearound to prevent excessive escape of gas past enlarged head 24 and the piston ring.

An igniter plug 34 is positioned rearwardly of power plug 18 in axial alignment therewith and in axial alignment with the barrel. As illustrated more clearlv in FIGS. 2 and 3, the igniter plug 34 includes a threaded igniter plug body 36, suitably formed of steel, and an igniter plug flange 38, also suitably formed of steel, which is secured to the body by means of screws 40. The igniter plug body 36 is substantially cylindrical and is provided with an axial center bore 42 for receiving electrode 44, suitably formed of steel, extending from the rearward end of igniter plug body to cylindrical center aperture 46 located at the forward end of the igniter plug body. Electrode 44 includes an enlarged cylindrical head 48 extending into center aperture 46 and spaced from the walls thereof. For example, a spacing 50 is provided Vbetween the end of center aperture 46 and head portion 48 so that explosive gas within the center aperture will be able to pass to the sides and behind enlarged head 48 whereby to prevent damage or movement of head 48 as a result of gas pressures during the operative explosion within the powder-actuated tool according to the present invention. Enlarged head 48 is drilled at one side of its forward face to receive one end of a heater Wire 52, such heater wire being silver soldered to head 48. The remaining end of heater Wire 52 is compressed between igniter plug -body 36 and igniter plug flange 38. In between the two ends of heater wire 52, the wire is preferably bent into a circuitous path such as an S-shaped curve, as illustrated, whereby to extend the length thereof. Heater wire 52 may be formed of a metal having a relatively high electrical resistance and stability at high temperatures, e.g. Nichrome, and in the particular embodiment has a diameter of approximately 0.025 inch.

In igniter plug 34, the outer portions thereof including igniter plug body 36 and igniter plug flange 38 provide one connection for heater wire 52, while electrode 44 provides the other connection. Therefore, electrode 44 is insulated from igniter plug body 36 by means of heat shrinkable insulated tubing 54 extending the full length of electrode 44 where electrode 44 passes through igniter plug body 36. Also, a fiber washer 56 surrounds the electrode 44 and insulated tubing 54 at the bottom of aperture 46. Furthermore, fiber washer 58 surrounds electrode 44 and insulated tubing 54, at the rearward end of the plug body, separating the plug body from an acorn nut contact 60 secured on the rearward threaded end of electrode 44. As will hereinafter become more evident, contact 60 comprises a contact means for closing an electric circuit to heater wire 52.

In the present embodment, the igniter plug and specifically igniter plug flange 38 extends forwardly to provide a cylindrical recess -62 in front of heater wire 52 for the reception of an explosive charge 64 in the form of a pellet or pill of uncased explosive material. For example, this charge may comprise a nitrocellulose pill, or similar explosive material, which is su-bstantially consumed when ignited whereby to generate a high gaseous pressure. In describing the pellet or pill as caseless, it is intended to indicate that the charge is not contained in a cartridge or the like, and is not provided with conventional primer or igniter means therewithin. The charge can, however, be provided with a thin consumable coating of plastic or -binder material adapted to improve handling thereof, and still be understood to be substantially caseless.

Referring again to FIG. 1, the igniter plug is insertable within a central chamber 66 in power plug 18 with charge 64 centrally disposed adjacent an aperture 68 in the power plug communicating with interior bore 20 of barrel 14. The charge 64 is thus disposed in a recess communicating with the barrel at the breech end thereof, that is, the end thereof opposite the muzzle. The recess wherein charge 64 is located may be within igniter plug 34 as shown, or such recess may alternatively comprise the chamber 66 in power plug 18, for example, as will hereinafter be more fully illustrated. In either event, when the tool is n loaded position, the exterior surface of charge 64 remote from the barrel is contacted by heater wire S2.

The plug body 36 of igniter plug 34 is threadably received Within breech block 70, the latter being slidably movable within the interior bore of breech portion 13 of the housing. The longitudinal movement of breech block 70 is, however, restricted by the shank end of screw 72 which extends into slot 74 in the breech block whereby the breech -block may move no farther forward than the position shown. The breech block 70 is urged into the position shown by spring 76 disposed between breech block 70 and a contactor block 78, positioned at the rearward end of the same -bore in breech housing portion 13.

Breech block 70 centrally supports longitudinal contact 80 within insulated bushing 82. Longitudinal contact 80 may comprise a Stud, bolt, or the like, and includes a first 4 contact end 83, aligned with contact 60 of igniter plug 34, and a second contact end 84 at the rearward end thereof. Contact end 84 is insulated from contactor block 78 -by means of an insulating washer 86, and contact end 83 is simlarly insulated from the contactor block by means of a compressible rubber washer 88. Longitudinal contact 80 is adapted to slide rearwardly through bushing 82 to a limited extent as contact end 83 is compressed against rubber washer 88. When longitudinal contact 80 is thus urged rearwardly, contact end 84 can make connection with third contact positioned within an aperture at the rearward end of breech portion 13 of the housing, and which is supported in this location by means of a screw 92 extending through stationary insulating support member 94 and rubber washer 96.

The tool is also provided with a hand grip 98 forming a continuation of housing breech portion 13, and a trigger 100 rotatable at pvot 102. The rearward surface of trigger 100 operates a microswitch 104 when the trigger is compressed rearwardly. The trigger is normally held in a forward position by means of biasing spring 106.

The tool is cocked by urging muzzle end 16 against a workpiece, for example a concrete wall. Barrel 14, together with power plug 18, igniter plug 34, and breech block 70 are urged rearwardly thereby, against the bias of spring 76, so that contact 60 makes connection with contact end 83 of longitudinal contact 80. Also, the outer periphery of the breech block 70 contacts the outer periphery of contactor block 78. Both breech block 70 and contactor block 78 are suitably formed of steel. Connection is therefore made therebetween for connecting the end of heater wire 52, as compressed between the igniter plug body 36 and igniter plug flange 38 of igniter plug 34. At the time the periphery of breech block 70 thus abuts the periphery of contactor block 78, longitudinal contact 80 is urged rearwardly, compressing rubber washer 88, and contact end 84 makes connection With third contact 90. Third contact 90 is urged toward rubber washer 96, compressing the same. Current is provided to the heater wire 52 through a pair of conductors 108 and 110 joined to third contact 90, via screw 92, and contactor block 78, respectively.

The source of current, hereinafter more fully described, is triggered to provide a current pulse when trigger 100 operates microswitch 104. The resulting pulse of current flowing in conductors 108 and 110 passes, for example, from conductor 108 through third contact 90, longitudinal contact 80, contact 60, and electrode 44 to a first end of heater wire 52. From heater wire 52, the current is then conducted through igniter plug 34, breech block 70, and contactor block 78 to conductor 110. As a result of this current, and the relatively high resistance of heater wire 52, the temperature of the lheater wire 52 rises rapidly and raises charge 64 to its ignition temperature. Charge 64 explodes, producing gases which pass through aperture 68 driving piston head 24 of captive piston 22 to the left. As the captive piston travels to the left, as illustrated in the drawing, along bore 20, slot 116 is exposed behind the piston to release excess gas pressure. Plunger 26 drives fastener device 30 into the workpiece.

The charge is thus ignited, and Vthe fastener device driven, without employing a cartridge or a complicated mechanical spring-actuated firing pin or the like. Moreover, less force is required by the operator to compress the tool against the workpiece to cock the tool than would be the case if a firing pin were employed, because of the elimination of the additional spring usually associated With the mechanical firing pin, which spring would be compressed in cocking a tool. The electrical ignition is simple, light weight, and incorporates the safety feature of requiring the tool to be cocked before complete electrical contact is made to Va triggerable source of current. Only a small amount of electric power is utilized in operating the tool according to the present invention.

'It should also be noted that the igniter plug is removable by unscrewing the same from breech block 70 whereby the igniter plug including the heater wire may eventually be conveniently replaced when it becomes necessary. When the igniter plug is secured within the tool as shown, and with the barrel 14 in operative position as illustrated, the igniter plug forms a gas seal in communication with the breech end of the barrel, and more specifically in contact with power plug 18, whereby rear- Ward escape of gas at this point is prevented.

The barrel 14 is normally prevented from moving to the left of the position illustrated in FIG. 1 by a detent 112 abutting shoulder 114 of slot 116 in the barrel portion 12 of the housing. However, for loading a new charge or pellet of explosive into the tool, the barrel 14 may be forced to the left, past detent 112, carrying therewith power plug 18. As the barrel is moved to the left, recess 62 is uncovered whereby a new charge of caseless explosive may be placed therein against heater wire 52. As the barrel is moved to the left, stop pin 118 engages shoulder 120 of piston head 24 urging the captive piston 22, which will have been forced to the left during the firing, back into the position wheren piston head 24 is disposed with the rearward portion thereof Within power plug 18. Stop pin 118 pressed into block 119 is normally held in place by a hinged -stop pin cover 122, the latter normally being secured in the position shown by screw 124. Screw 124 may be removed, and stop pin cover 122 swung downwardly for removing =block 119 and pin 118 in dismantling the tool, for example. Then barrel 14 including captive piston 22 and power plug 18 may then be slid to the left out of the tool. Pin 118 and Vblock 119 are slidable to the right along cover 122 when the tool is cocked.

FIGS. 5, 6, and 7 illustrate three alternative triggerable circuits for providing current to the powder actuated tool of the present invention. Referring to FIG. 5, a positive terminal of a battery 126 is connected to one end of heater wire 52 through fuse 128 and contacts 130, while the negative terminal of the same battery is connected to the other end of heater wire 52 through trigger-operated switch 104a (corresponding to switch 104 in FIG. 1) land contacts 132. Contacts 130 and 132 represent the central and outer contacts which make connection when the tool is cocked. For example, contacts 130 may inclusively represent contacts 60, 80, and 90 in FIG. 1, while contacts 132 similarly represent the outer peripheres of breech block 70 and contactor block 78. Thus, when the trigger is depressed, trigger-operated switch 10411 is closed, completing the circuit to heater wire 52, if the tool is cocked such that contacts 130 and 132 are closed. Ignition normally occurs immediately, and the recoil of the tool acts to open contacts 130 and 132, i.e. since the tool including contactor block 78 will then move rearwardly relative to breech block 70. Therefore, a current of short duration will pass through heater wire 52 raising the same to a high temperature. The heater wire, being relatively large, conducts appreciable current for a short period of time for the purpose of applying heat to an appreciable part of the charge, thereby insuring good gnition of the same. In the event that immediate explosion within the tool and consequent recoil does not occur, fuse 128 operates to disconnect the battery to |avoid undue battery drain and burn-out of the heater wire.

Another type of triggering circuit is illustrated in FIG. 6 where battery 126 charges capacitor 134 through resistor 136 and through the normally closed contacts of trigger-operated switch 104b (corresponding to switch 104 in FIG. 1). Then, when the tool is cocked and the trigger 100 operates switch 104b to its second position, capacitor 134 discharges rapidly through contacts 130 and 132 and heater wire 52 rapidly raises the latter to a high temperature for igniting the charge. In this case, the capacitor can supply only a limited current until discharged, and no further demand is made on the battery until subsequent triggering, nor can burn-out of the heater coil take place.

In the circuit of FIG. 7 battery 126 charges smaller capacitor 134 through resistor 136 in the normal position of trigger-operated switch 104b (corresponding to switch 104 in FIG. 1). When switch 104b is closed to its alternate position by operation of trigger 100, the charge on the capacitor is effective to turn on NPN switching transistor 138, which then turns on PNP switching transistor 140, wherein the latter transistor may be of the power type designed to carry appreciable current. Providing the tool is cocked, current will flow from battery 126 through contacts 130 and heater wire 52, and then through contacts 132 and transistor back to the battery. Current will flow through the heater wire only so long as the charge in capacitor 134 causes the transistors to remain in their conductive state. When capacitor 134 discharges, transistor 140 will turn off, thereby discontinuing the flow of current. However, the transistor 140 may be kept nearly full on until a predetermined time in the discharge cycle of capacitor 134.

The tool including the FIG. 7 circuit, for example, possesses several advantages. The circuit is self-protected in that once the trigger-operated switch 104b is actuated, the current pulse provided will be of substantially the same magnitude and duration whether the trigger is held in for a short or a long period of time. Thus, heater wire overload is prevented. Also, in order to prevent overload, the R.M.S. value of current in the heater wire is limited, as in the instance when the trigger is operated repeatedly. The latter result is accomplished by preventing repeated current pulses from following one another too closely. The time constant of capacitor 134 and resistor 136 is arranged so as to require a predetermined interval before sufiicient charge is again provided on capacitor 134 for operating transistor 138. However, current through switch 104b to small capacitor 134 is very low in value thereby extending switch life, with the switch ordinarily breaking no current, since capacitor 134 usually fully discharges through the transistor. The overall cost of the FIG. 7 circuit is quite low in comparison with other possible systems performing all the same functions.

The battery and circuit employed according to any one of FIGS. 5, 6, and 7 versions may be incorporated in the housing of the tool if so desired, or the conductors of microswitch 104 as well as conductors 108 and 110 may be brought out of the tool to a separate power package.

Another embodiment of a powder actuated tool in accordance with the present invention is illustrated in partial cross section in FIG. 4, wherein similar components are referred to with like reference numerals. The loading action on this type of tool is somewhat different from that employed in FIG. 1. In the FIG. 4 embodiment the breech portion 13 of the housing is turned around the longitudinal axis of the tool, with respect to barrel 14, such that lugs 142 are aligned with matching slots (not shown) in the breech block 70, whereby housing portion 13 may be moved to the right in FIG. 4. The tip of screw 146, which is mounted in the breech portion of the housing, extends into a longitudinal slot 148 in shaft 144 secured to the barrel portion, whereby slot 148 limits the longitudinal rearward movement of the breech portion. When housing portion 13 is moved to the right, igniter plug 34 will be clear of the power plug 18, and portion 13 of the housing may be rotated upon shaft 144, with the tip of screw 146 being received in an arcuate groove 149. The remainder of the forward part of the tool is similar and operates in a manner similar to the embodiments already described. The barrel 14 again receives a captive piston (not shown), the piston head of which is adapted to be received within power plug 18 proximate aperture 68 next to charge 64. The further details of the Construction of the forward portion of this type of tool are shown in Neighorn Pat. 3,3l9,862, is-

sued May 16, 1967, entitled Powder Actuated Tool, and assigned to the assignee of the present invention.

In the FIG. 4 embodiment, charge or pellet 64 of uncased explosive is received directly within chamber 66 in power plug 18, wherein such chamber provides for the recess receiving the charge and communicating with the barrel of the tool through the aforementioned aperture 68. Igniter plug 34 is threadably received in breech block 70 and again supports an insulated center electrode 44 connected to heater wire 52, while the remaining end of the heater wire is joined to the exterior of igniter plug 34. Contact 60 is threadably secured to electrode 44, with a periphery of the contact 'being insulated from igniter plug 34 with fiber washer 150. A second contact 152, in aligned relation with contact 60, is centrally located within insulating block 154 wherein the latter is urged by spring 155 against the inner periphery of breech block 70. Contact 152 is thus held in contact with contact 60 for any longitudinal position of the breech block and igniter plug. An annular contactor block 156 is positioned within breech portion 13 of the housing, in alignment with and rearward of the outer periphery of breech block 70.

When the muzzle end 16 of the tool is urged against a workpiece, the barrel 14 moves power plug 18, igniter plug 34 and breech block 70 toward contactor block 156, compressing spring 155. Contact is made between the outer periphery of breech block 70 and the outer periphery of contactor block 156 whereby to complete a circuit between conductor 110 and the heater wire 52. Thus, breech block 70 and contactor block 156 suitably form the contacts 132 referred to in the schematc diagrams of FIGS. 5, 6, and 7. The operation of the FIG. 4 tool and the connection thereof with the triggerable circuit for supplying current is otherwise substantially the same as in the FIG. 1 embodiment.

The tools illustrated herein have been of the captive piston type, wherein the captive piston drives the fastener device. Such piston has high momentum but relatively low velocity. However the present invention is also applicable to high velocity type tools where the gaseous pressure from the explosion of the electrically ignited charge or pellet of caseless explosive is used directly to drive the fastener device, without the use of an intermediate captive piston.

While we have shown and described preferred embodiments of our invention it will be apparent to those skilled in the art that many changes and modifications may be made without departing from our invention in its broader aspects. We therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

We claim:

1. In a powder actuated tool including a barrel for receiving a drivable fastener device,

means cooperable with said barrel at a first end thereof for driving said fastener device from the opposite end of said barrel, said means being provided with a recess co'mmunicating with the first end of said barrel for receiving a charge of caseless explosive, and electrical heater means adapted to contact the external surface of said charge of caseless explosive in said recess,

and triggerable means for providing a current through said heater means for raising the temperature of said charge to a value for igniting said explosive and ejectin-g said fastener device from said barrel.

2. The device according to claim 1 wherein said means cooperable with said barrel for driving said fastener in` cludes an igniter plug provided with said heater means, said igniter plug being longitudinally movable within said powder actuated tool in response to compression of the forward end of said tool against a workpiece,

and an electrical contact movable with said igniter plug for making a connection between said heater means and said triggerable means only when said tool is urged in compression against said workpiece,

3. The device according to claim 2 wherein said igniter plug is provided with said recess, said heater means being located therewithin.

4. The device according to claim 1 further including a captive piston slidably received in said barrel 'between said fastener device and said first end of said barrel, said piston being adapted to move axially toward said fastener device in response to gaseous pressure for driving said fastener device when said charge of caseless explosive is ignited.

5. The device according to claim 1 wherein said electrical heater means comprises an electrical conductor positioned for contacting said charge of caseless explosive at the side thereof remote from said barrel, said conductor having a resistance adapting the same to produce a rapid rise in temperature when current is passed therethrough.

6. The device according to claim 5 wherein the conductor is formed of Nichrome wire.

7. A powder actuated tool co'mprising:

a housing having a barrel portion and a breech portion,

a barrel for receiving a drivable fastener device, said barrel being slidably received within the barrel portion of said housing,

a breach block slidably received within the breech portion of said housing in longitudinal alignment with and behind said barrel,

an igniter plug secured to said breech block and provided with electrical heater means adapted to contact the external surface of a charge of caseless explosive insertable between said igniter plug and said barrel,

means for providing electric current,

and electrical contact means movable with said breech block for making connection between said heater means and said means for providing electrical current in response to sliding movement of said barrel in the direction of said breech block.

8. The device according to claim 7 further including a stationary contactor block within said breech portion of said housing aligned with said breech block and towards which said breech block is urged by sliding movement of said barrel,

and wherein said electrical contact means comprises an electrical contact on said igniter plug, and an aligned electrical contact on said contactor block.

9. The apparatus according to claim 7 further including a power plug secured to said barrel having a chamber for receiving at least a portion of said igniter plug and said charge of caseless explosive, with said heater means on said igniter plug contacting said caseless explosive, said power plug having an aperture communicating from said Chamber to said first end of said barrel.

10. The device according to claim 8 including a compression spring located in said breech portion of said housing substantially between said breech block and said contactor block for normally urging said breech block in the direction of said barrel.

11. The apparatus according to claim 7 wherein said means for providing an electric current includes an electrical switch, a trigger means for operating said electrical switch, and a circuit operable by said switch for supplying current to said heater means through said electrical contact means.

12. The device according to claim 7 wherein said igniter plug includes a metal body threadably secured to said breech block and removable from said breech block, and a center conducting electrode insulated from said body, said heater means comprising a resistance wire connected at an end of said igniter plug between said metal body and said center conducting electrode.

13. The apparatus according to claim 11 wherein said circuit includes a transistor amplifier operable in reponse 9 10 to operation of said switch for completing a conductive References Cited path for supplying current to said heater means through UNITED STATES PATENTS said electrical contact means.

14. The apparatus according to claim 13 wherein said 2931,039 4/1960 Henning et al' 227 11 conductive path is completed between Collector and r) 3251316 5/1966 Brqske 227 10 XR emitter electrodes of a transistor of said transistor 3=319 862 5/1967 Nelghom 227 10 amplifier. GRANVILLE Y. CUSTER, JR., Primary Exarnner 

